Disc Bimetallic Thermostats


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Intro

The purpose of this post is to go over our Disc Bimetallic Thermostat (aka temperature regulator/thermal switch/bimetal cutout) product offerings.

Although the product design is simple to understand, we do get some questions on them.

A fair amount of calls also come through from customer service with customers claiming that the parts are defective.

Bimetal Strips/Discs

The crucial component of our thermal switches is a bimetal strip. Common bimetal materials are steel/copper and steel/brass which are welded, brazed or riveted together.

As the two metals have different coefficients of expansion any changes in temperature will cause the bimetal strip bend one way or the other (depending on an increase or decrease in temp).

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Disc Bimetallic Thermostats

Bimetal Cutouts utilize a bimetallic disc in order to convert temperature change to mechanical displacement. To do this the bimetallic disc needs to be combined with contacts, terminals and a housing.

As is shown by this diagram of a Cantherm product, the bimetallic disk moves an actuator pin that separates or connects the contacts.
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The drawing above illustrates, the construction of the R-series bimetal cutouts used in this booth.

The diagram below shows one of the through hole bi-metal cutouts from Honeywell. It is not the exactly the same design but the concept is the same.
As the bimetal disc heats and deforms the contacts are separated.
Bimetal cutouts are relatively expensive ($5-$40 single piece depending on size and mfr). But, they are very easy to use temperature dependant switches.
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Applications

Here are some common applications where bi-metal cutouts may be found. * HVAC equipment * Copy machines * Medical equipment * Various small or large home appliances * Motors and transformers * Fire/heat detectors * Power supplies

Selection Criteria

There are a handful of specifications that need to be considered when selecting a Disc Bimetallic Thermostat.

  • Actuating temperature
  • Ambient operating temperature
  • Contact rating
  • Life Expectancy
  • Humidity
  • Termination and Mounting

Actuating Temperature

  • The actuating temperature is simply the temperature that the thermostat opens or closes the circuit. It is expressed as the open or close temperature. The thermostat will be advertised as normally open or closed in respect to room temperature.
  • The actuating or trip temperature is NOT the same as the reset temperature.
    For Example See Also:
  • The actuating temperature will have specified tolerance. This tolerance is specified in °C/°F and is listed differently depending on the mfr.
    • Cantherm for example lists a average tolerance for one whole series, while Sensata lists the tolerance at a few different temperature ranges in one series. Honeywell is pt# specific with their information.
  • The lower the actuation temperature the smaller the range in actuation temperature. But keep in mind that a 3°C tolerance at 10°C is a lot more noticeable than a 6°C tolerance at 100°C.

FAQs
Why do bimetal cutouts not reset immediately after falling below their actuation temperature?

  • The construction of the bimetal disc causes this issue. The bimetal disc is designed so that it is always dome shaped rather than flat.
  • As the temp changes the amount of force, due to the different coefficients of expansion, will build until it is great enough pull the dome from one face to the other.
  • As an example, the discs are similar to the WURTH mint candy lid included here. With enough force the dome will move from one side to the other but the lid never remains in a flat position
  • It makes sense that the temperature has to drop considerably below the actuating point in order to build up enough force to return the dome to its original position.
  • This design is necessary to provide an instant open/closure of the contacts. If the contacts moved together/apart slowly along with a gradual temperature change, arcing would quickly destroy the contacts.

What is the Reset Temperature?

  • This is understandable since some of these parts can reset anywhere from 5~50°C below the actuating temperature.
  • The only way this statement helps is by giving us documentation to help explain that these parts are not supposed to reset as soon they fall below their actuation temperature.

Ambient Temperature

Like anything else Disc Bimetallic Thermostats can be damaged by excessive temperatures.

In some applications the bimetal cutout is a short distance from the major heat source. So even after the circuit opens the existing heat continues to radiate/conduct and further heat the thermostat.

If the thermostat is being used to turn on a cooling system the same problem can arise. As the temperature continues to rise, it may take a minute or two for a cooling mechanism to catch up and then cool the circuit below the actuating temperature.

In most applications this is not an issue because bimetal cutouts typically have high operating temperatures. The cutouts used in this booth have a continuous maximum operating temperature of 175 C at full load.

Contact Rating

Like any mechanical switch or relay, contact ratings are an important specification.

Are the current and voltage rating capable of handling the customers needs? Things to consider

  • What are the current/voltage requirements of the load?
  • Is the application AC or DC?
  • Is the load resistive or inductive?
  • Is the application going to be used in pilot duty (ex is the thermal cutout going to control a relay rather than the actual load)?

Life Expectancy

How long does the thermostat need to last?

  • The R53 series thermostats have been tested up to 100K cycles in pilot duty (R53’s are used in this booth). With a load of 12VDC at 15A the life cycle is only guaranteed to 20K cycles

Bimetal cutouts will experience some drift in actuation temperature. This drift is typically seen between 10K and 20K cycles with small or large loads. The amount of drift is usually less than 5°C in either direction

Humidity

No big surprise here. Excessive humidity can negatively effect unsealed bimetal thermostats. Thermostats like the Cantherm series used in this booth are not sealed. Excessive humidity can cause * Electrical arcing * Oxidation on the bimetal strip * Deformation of the polymeric material

We also carry some sealed bimetal thermostats that withstand excessive humidity and automated soldering/washing processes.

Physical Dimensions

There are multiple packages offered in the bi-metal thermostat line. As contact ratings go up the thermostats often become larger and change from PCB to Chassis mount.

Defective parts

With disc bimetal thermostats the most common complaint is that the part is supposed to be NC or NO but instead appears to be open or closed (the opposite).

When dealing with low temperature thermostats 15°C ~ 30°C the parts will trip when traveling in a hot vehicle or sitting in a hot warehouse. When the customer puts the bimetal cutout into their design it is tripped even though the current ambient temperature is around 10°C ~ 25°C.

At this point simply explaining how travel/storage temperatures will trip thermostat is easy enough. The fix is easy to. Often we suggest to our customers to throw the parts in a refrigerator for while so they can reset.

This is why it is important to be aware of the differential between the trip and reset points. This sometimes this can turn out to be an issue for the customer. If a simple bimetal cutout will not work for the customer because of the temperature differential they will have to go to a more involved product design. Whether it be some sort of thermistor, thermocouple, semiconductor, etc the customer will have to do a little more involved circuit design, as they will know no longer be working with a simple switch.

While many of the “defective part” calls coming in are dealing with low temperature versions, this same issue can arise at higher temperatures. There have been instances where a customer may try to use one of these devices to closely monitor a working temperature.

Example. They may use bimetal thermostat in order to turn on an indicator when 110°C is surpassed with the belief that the indicator will turn back off as soon as the temperature drops below 110°C. However, as we know the temperature could drop below 100°C with the unaware operator believing that they are at 110°C or greater.

Other Thermal Switch Options

We also list a couple other thermal switch options. There are thermistor based thermal switches (both NTC and PTC). As they are thermistors the operating current has to be very low in order to not significantly effect the switching temperature of the thermistors. These specialized thermistors are priced similar to the smaller bimetal disc thermistors.

Looking to stretch a dollar

  • Thermal Links/Fuse. These products will also open at specific temperature. These parts are much less expensive (sub $1) but are one time use only.
  • The two on the left use a fusible alloy that melts a specific temperature. The part with the green epoxy uses a wax material to hold spring contacts in place. As the tripping temperature is reached the wax melts and the contacts spring apart.